The present disclosure relates to a control device and a control method for a continuously variable transmission mounted on a vehicle.
There has hitherto been known a control device for a continuously variable transmission that can steplessly vary output rotation of a drive source, the control device changing the continuously variable transmission stepwise (in a stepped manner) among a plurality of shift speeds set in advance (see Japanese Patent Application Publication No. 2008-309229, for example). When an automatic speed change mode is set, the control device changes the speed ratio of the continuously variable transmission stepwise from a first speed to a seventh speed using seven groups of speed ratio lines, namely a group of first speed ratio lines (first speed group) to a group of seventh speed ratio lines (seventh speed group), each group including three speed ratio lines with different speed change characteristics.
There has also been known a control device for a continuously variable transmission, which selects one of a normal speed change mode and a pseudo stepped upshift mode in accordance with the magnitude of an acceleration request from a driver, and which controls the speed ratio of the continuously variable transmission on the basis of the selected mode (see Japanese Patent Application Publication No. 2010-007749, for example). When the pseudo stepped upshift mode is selected to control the speed ratio, the control device judges that an acceleration request from a driver is provided in the case where the operation speed of a throttle valve is equal to or more than a predetermined threshold, and performs an upshift at predetermined time intervals or at time intervals set to be short in the case where at least one of the vehicle speed and the operation amount of the throttle valve is small. Consequently, an upshift is performed at a lower engine rotational speed as the magnitude of the acceleration request from the driver is small, and a travel state in which the engine rotational speed rises in conjunction with a rise in vehicle speed occurs repeatedly.
There has additionally been known a control device for a continuously variable transmission, in which a target input rotational speed for the continuously variable transmission is varied stepwise to perform a stepwise upshift when the target input rotational speed reaches an upshift determination value and perform a stepwise downshift in accordance with an increase in accelerator operation amount (see Japanese Patent Application Publication No. 2013-200003, for example). The control device calculates the target input rotational speed by adding a vehicle speed correction value, which is set to become larger as the vehicle speed becomes higher, and an accelerator operation amount correction value, which is set to become larger as the accelerator operation amount becomes larger, to a base rotational speed that serves as a reference. In order to vary the target input rotational speed stepwise, in addition, the control device updates the value of the base rotational speed at a particular timing using the accelerator operation amount which varies stepwise in calculating the accelerator operation amount correction value.